【JavaScript】手写Promise

手写一个简单的Promise可以包含以下几个状态和方法：pending、fulfilled、rejected，以及then、catch方法。

class SimplePromise {
  constructor(executor) {
    this.status = 'pending'; // 初始状态为pending
    this.value = undefined; // 保存resolve的值
    this.reason = undefined; // 保存reject的原因
 
    const resolve = (value) => {
      if (this.status === 'pending') {
        this.status = 'fulfilled';
        this.value = value;
      }
    };
 
    const reject = (reason) => {
      if (this.status === 'pending') {
        this.status = 'rejected';
        this.reason = reason;
      }
    };
 
    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }
 
  then(onFulfilled, onRejected) {
    if (this.status === 'fulfilled') {
      onFulfilled(this.value);
    }
    if (this.status === 'rejected') {
      onRejected(this.reason);
    }
  }
 
  catch(onRejected) {
    this.then(undefined, onRejected);
  }
}
 
// 使用示例
const promise = new SimplePromise((resolve, reject) => {
  // 异步操作
  setTimeout(() => {
    // resolve(42); // 可以更改为其他操作
    // reject('Operation failed');
  }, 1000);
});
 
promise.then(
  (value) => console.log('Resolved:', value),
  (reason) => console.log('Rejected:', reason)
);

这个简易版本的SimplePromise类实现了then和catch方法，以及内部的resolve和reject逻辑。这个实现没有考虑链式调用和并行处理，但足以演示基本的Promise行为。